Chiral α-Amino Acid-Based NMR Solvating Agents

Article abstract of DOI:10.1002/hlca.202000081

Four new chiral α-(nonafluoro-tert-butoxy)carboxylic acids were synthesized from naturally occurring α-amino acids (alanine, valine, leucine and isoleucine, respectively), and tested in ¹H- and ¹⁹F-NMR experiments as chiral NMR shift reagents. The NMR studies were carried out at room temperature, using CDCl₃ and C₆D₆ as solvents, and (RS)-α-phenylethylamine and (RS)-α-(1-naphthyl)ethylamine as racemic model compounds. To demonstrate the applicability of the reagents, the racemic drugs ketamine and prasugrel were also tested.

Full text of DOI:10.1002/hlca.202000081

HELVETICA
chimica acta
Accepted Article
Title: Chiral α-Amino Acid-Based NMR Solvating Agents
Authors: Anikó Nemes, Tamás Csóka, Szabolcs Béni, Zsófia Garádi,
Dénes Szabó, and József Rábai
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Helv. Chim. Acta 10.1002/hlca.202000081
Link to VoR: https://doi.org/10.1002/hlca.202000081
www.helv.wiley.com

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
Chiral α-Amino Acid-Based NMR Solvating Agents
,a
a
b
b
a
a
Anikó Nemes,* Tamás Csóka, Szabolcs Béni, Zsófia Garádi, Dénes Szabó, and József Rábai
a
Institute of Chemistry, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117, Budapest, Hungary, neagaft@caesar.elte.hu
b
Department of Pharmacognosy, Semmelweis University, Üllői út 26, 1085, Budapest, Hungary
Dedicated to Prof. Antonio Togni on the occasion of his 65th birthday
Four new chiral α-(nonafluoro-tert-butoxy)carboxylic acids [(R)-1, (R)-4, (R)-5, (2R,3S)-6] were synthesized from naturally occurring α-amino
acids (alanine, valine, leucine and isoleucine, respectively), and tested in H and 19F NMR experiments as chiral NMR shift reagents. The NMR
1
studies were carried out at room temperature, using CDCl
3
and
C
6
D
6
as solvents, and (RS)-α-phenylethylamine and (RS)-α-(1-
naphthyl)ethylamine as racemic model compounds. To demonstrate the applicability of the reagents, the racemic drugs ketamine and
prasugrel were also tested.
Keywords: amino acids • carboxylic acids • enantioselectivity • chiral pool
interaction, π-π interaction between aromatic rings or steric
Introduction
repulsion.[15] The most widely used salt forming chiral solvating
Since most of the active pharmaceutical ingredients (APIs) are
optically active molecules, pharmaceutical industry needs simple, fast
and accurate analytical methods for determining enantiomeric ratios.
Chiral analytical techniques include (1) chromatography (GC [1], HPLC)
with the use of chiral stationary phases [2] or achiral stationary phases
after chiral derivatization,[3] (2) capillary electrophoresis (CE),[4] and (3)
chiral spectroscopies.[5] Disadvantage of chromatography resides in
the lengthy optimization processes, while spectroscopic methods,
such as ECD, VCD and ROA require special equipment, therefore they
are relatively rarely used as routine measurements.
agents are mandelic acid derivatives, such as the Mosher’s acid
MTPA)[16,17,18] and aromatic benzyl alcohols, such as Pirkle’s
(
[
1
9
,
2
0
]
1
a
l
c
o
h
o
l
.
B
o
t
h
r
e
a
g
e
n
t
s
s
h
o
w
a
r
o
u
n
d
0
.
0
2
0
-
0
.
0
7
5
p
p
m
Δ
δ
i
n
H
NMR, and 0-0.010 ppm in 19F NMR as chiral solvating agents.
In our previous work we showed, that α-(nonafluoro-tert-
butoxy)carboxylic acids, based on lactic acid (R)-1, mandelic acid
(RS)-2 and 3-phenyllactic acid (R)-3, display good chiral
discrimination properties towards chiral amines. The chemical shift
differences of these compounds are comparable with the literature
values of other shift reagents, namely 0.011-0.190 ppm in 1H NMR
NMR spectroscopy offers an advantageous possibility for chiral
discrimination,[6-9] because NMR instruments are usually available for
routine structure determinations. The ee determination by NMR
spectroscopy is based on diastereomer formation, either as (1)
derivatization in a separate step before the measurement, or (2) in
situ complex formation (using lanthanide shift reagents or chiral
solvating agents). These latter procedures have the advantage of fast
optimization and data processing, as well as lacking racemization and
purification.
and 0.006-0.063 ppm in 19F NMR in case of (RS)-α-phenylethylamine,
as test analyte. We also observed chemical shift differences in 19
F
NMR in the case of racemic ephedrine (0.008-0.010 ppm).[21,22] To
extend these experiments, we synthesized four new chiral carboxylic
acid derivatives with similar structures, starting from natural α-amino
acids. In order to introduce the nonafluoro-tert-butoxy moiety, the
amino group was replaced by stereospecific nucleophilic substitution.
In this paper we present the synthesis and chiral discrimination
studies of carboxylic acids (R)-1, (R)-4, (R)-5 and (2R,3S)-6 (Figure 1).
The complex formation required for the stereodiscrimination can be
based on host-guest interactions, e.g. cyclodextrins (CDs) and crown
ethers. In the case of CDs the chemical shift difference (Δδ) of the
analyte 1H resonances are in the 0.005-0.120 ppm range, typically
0
.02-0.05 ppm.[10-12] Using crown ethers as CSA, the hydrogens of the
analyte show around 0.009-0.250 ppm chemical shift differences.[13,14]
More often the complex formation takes place using diverse donor-
acceptor interactions e.g. hydrogen bonding, dipole-dipole
1
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
In our NMR experiments diastereomeric salt formation was
investigated between (R)-1, (R)-4, (R)-5 and (2R,3S)-6 carboxylic acids
and racemic α-phenylethylamine (PEA) and α-(1-naphthyl)ethylamine
(NEA) as test analytes. In apolar solvents (CDCl
3
and C
6
D ) the
6
diastereomeric salts are present as tight ion pairs, thus the recorded
chemical shifts are weighted averages of the free and the protonated
amines, which are present in the tight diastereomeric complex. The
amounts of the enantiomers can be calculated from the integrals of
the signals in 1H and 19F NMR spectra. Our first attempt was to
observe the enantiorecognition on the 1H and 19F nuclei of the
Figure 1. Structure of α-(nonafluoro-tert-butoxy)carboxylic acids.
racemic
butoxy)carboxylic acids (1, 4, 5, 6) as chiral solvating agents. Spectra
were recorded in 54 mM using CDCl as solvent (Table 1, Figure 2). In
the cases of (RS)-PEA as analyte, the methyl hydrogens of the amine
doublets, ~ 1.6 ppm) did not show measurable differences in the
α-phenylethylamine,
using
α-(nonafluoro-tert-
Results and Discussion
3
The starting materials of the synthesis of α-(nonafluoro-tert-
butoxy)carboxylic acids are the natural amino acids alanine, valine,
leucine and isoleucine. The first step is diazotation of amino group
followed by hydrolysis to yield the optically active α-
hydroxycarboxylic acids.[23] In this reaction double inversion of
configuration occurs in a stereospecific way. The first reaction step is
a diazotation, which is followed by a lactone formation, as the
carboxyl group attacks the α-carbon atom, therefore the overall
process results in the inversion of configuration.[24] Then the formed
α-lactone hydrolyzes in situ also with inversion.[25] The obtained
(
chemical shifts of the 1H NMR resonances. However, the methine
hydrogens (quartets, ~ 4.25 ppm), which are attached directly to the
stereogenic carbon atom, exhibited distinguishable chemical shift
difference (Δδ) in the range of 0.035-0.045 ppm for (R)-1, (R)-4 and
(R)-5. Concurrently (2R,3S)-6×(RS)-PEA has broad signals in the
spectrum due to solubility problems, thus the enantiorecognition
phenomena cannot be proved under these conditions. It is worthy to
note that these quartet signals were overlapping, thus exact
determination of the relative amounts of enantiomers was not
possible.
carboxylic acids were reacted with SOCl
to give the methyl α-hydroxycarboxylate intermediates (S)-7, (S)-8,
S)-9 and (2S,3S)-10. Based on our previous experiments the ether
bonds were formed with nonafluoro-tert-butanol under Mitsunobu
2
in the presence of methanol,
(
[
26,27,28,29]
reaction conditions
inversion of configuration. In the last step the ester protecting group
was hydrolyzed with NaOH in MeOH-CH Cl solvent mixture, to give
the target α-(nonafluoro-tert-butoxy)carboxylic acids (R)-1, (R)-4, (R)-
and (2R,3S)-6 (Scheme 1).
which took place with complete
2
2
5
Scheme 1. Synthesis of optically active α-(nonafluoro-tert-butoxy)carboxylic acids.
2
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
In a second set of experiments the same measurements were
performed using C
methyl hydrogens (~ 1.5 ppm) have significant Δδ in the range of
.025-0.035 ppm and due to the smaller peak widths of these
6
D
6
as solvent (Table 1, Figure 3). In these cases,
0
doublets, their integrals can be used for enantiomeric ratio
determination. Surprisingly, the methine hydrogens directly attached
to the stereogenic center (~ 4.05 ppm) did not give significant Δδ,
only (R)-1×(RS)-PEA has
a
small 0.010 ppm anisochrony.
Unfortunately, 19F NMR measurements failed to show Δδ in any of
the above cases.
Table 1. Chemical shift differences (Δδs) of (RS)-PEA hydrogens.
Solvating
agent
Δδ (CH) /
3
Δδ (CH ) /
Solvent
ppm
ppm
(
(
(
R)-1
R)-4
R)-5
CDCl
CDCl
CDCl
CDCl
3
3
3
3
0.045
0
0
0.035
0.040
0
Figure 3. Partial 1H NMR spectra of α-(nonafluoro-tert-
(
(
2R,3S)-6
broad signals
0
butoxy)carboxylic acids with (RS)-PEA in C
6
6
D .
(
(
(
R)-1
R)-4
R)-5
C
C
C
C
6
6
6
6
D
D
D
D
6
0.010
0.035
0.030
0.035
0.025
6
6
6
0
0
0
Similar to the experiments presented above, NMR measurements
were also performed with racemic α-(1-naphthyl)ethylamine as the
analyte. In CDCl solution using (R)-4 and (2R,3S)-6 as chiral solvating
3
2R,3S)-6
agents the methyl hydrogens (~ 1.7 ppm) of the (RS)-NEA showed
distinguishable chemical shift differences (0.015 and 0.020 ppm),
while the methine hydrogens (~ 5.15 ppm) resulted in smaller Δδ
values (0.015-0.030 ppm) than in the case of (RS)-PEA (Table 2,
Figure 4). Unfortunately, diastereomeric complexes of the carboxylic
acids (1, 4, 5, 6) and (RS)-NEA have low solubility in the apolar C
6
6
D ,
thus NMR spectra could not be evaluated in this solvent. Under these
conditions 19F nuclei also did not show difference in their chemical
shifts.
3
Table 2. Chemical shift differences (Δδs) of (RS)-NEA hydrogens in CDCl .
Solvating agent
Δδ (CH) / ppm
3
Δδ (CH ) / ppm
(
(
(
R)-1
R)-4
R)-5
0.030
0.015
0
0.015
broad signals
0.020
broad signals
0.015
(
2R,3S)-6
Figure 2. Partial 1H NMR spectra of α-(nonafluoro-tert-
butoxy)carboxylic acids with (RS)-PEA in CDCl
3
.
For further exploration of the potential of using (R)-1, (R)-4, (R)-5 and
2R,3S)-6 as chiral NMR shift reagents their applicability has also
(
been tested using the secondary amine ketamine and the tertiary
amine prasugrel as active pharmaceutical ingredients, respectively.
3
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
Table 4. 1H NMR chemical shift differences (Δδs) of (RS)-prasugrel in the
Both racemic compounds showed the chiral recognition phenomena
1
3
presence of various α-(nonafluoro-tert-butoxy)carboxylic acids in CDCl .
in H NMR measurements.
1
First the H NMR spectra of the diastereomeric salts of ketamine were
Solvating
agent
Δδ (H ) /
Δδ (H ) /
Δδ (H ) /
Δδ (H )/
a
b
c
d
ppm
ppm
ppm
ppm
recorded in CDCl . Significant chemical shift differences were
3
observed in the case of the methylene resonances of the
cyclohexanone ring adjacent to the chiral center 0.011-0.015 ppm.
Small Δδs (0.002-0.004 ppm) were recognized for the N-methyl
hydrogens (Table 3, Figure 4). In order to obtain better resolution
between the hydrogen signals of the diastereomeric complexes the
(
(
(
R)-1
R)-4
R)-5
0.035
0.042
0.050
0.041
0.060
0.064
0.086
0.066
0.008
0.009
0.014
0.010
0.007
0.008
0.003
0.010
(
2R,3S)-6
more apolar solvent C
6
D
6
was also tested. Unfortunately, these
experiments failed, due to the low solubility of the compounds.
Table 3. 1H NMR chemical shift differences (Δδs) of (RS)-ketamine in the
presence of various α-(nonafluoro-tert-butoxy)carboxylic acids in CDCl
3
.
Solvating agent Δδ (H ) / ppm Δδ (H ) / ppm
a
b
(
(
(
R)-1
R)-4
R)-5
0.015
0.011
0.014
0.011
0.002
0.004
0.003
0.004
(
2R,3S)-6
Figure 5. Selected 1H NMR resonances of (RS)-prasugrel (bottom)
and the same resonances after the addition of equimolar amounts of
various α-(nonafluoro-tert-butoxy)carboxylic acids in CDCl .
3
The 1H NMR spectra of prasugrel along with the new NMR shift
reagents were also recorded in C . In these cases, the magnitude of
the observed chemical shift differences was comparable with those of
Δδs measured in CDCl . Due to some spectral overlap in C the Δδs
were observed for the following resonances: H , H , H (see Figure 6).
It is worthy to note that in C even H resonances exhibit significant
anisochrony 0.004-0.008 ppm (separated by 5 bonds from the chiral
center) compared with that of registered in CDCl . The chemical shift
anisochronies of H were 0.046-0.077 ppm, somewhat smaller, than
that of observed in CDCl also
. The aromatic 1H resonance H
exhibited 0.018-0.037 ppm anisochrony (Table 5, Figure 6).
6
D
6
Figure 4. Selected 1H NMR resonances of (RS)-ketamine (bottom)
and the same resonances after the addition of equimolar amounts of
3
6
D
6
various α-(nonafluoro-tert-butoxy)carboxylic acids in CDCl .
3
e
f
g
6
D
6
f
Then the optically active α-(nonafluoro-tert-butoxy)carboxylic acids
were used as CSAs for the racemic prasugrel in CDCl
. 1H resonances
relatively close to the chiral center show substantial anisochrony
3
3
b
0.035-0.050 ppm for H
a
and 0.060-0.086 ppm H , respectively. The
b
3
g
1
aromatic H resonances separated by four covalent bonds from the
stereocenter (H
c
and H ) still exhibit recognizable chemical shift
d
differences (Table 4, Figure 5).
4
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
1
Table 5. H NMR chemical shift differences (Δδs) of (RS)-prasugrel in the
Experimental Section
6 6
presence of various α-(nonafluoro-tert-butoxy)carboxylic acids in C D .
Solvating
agent
Δδ (H
b
)
Δδ (H
f
)
g
Δδ (H )/
General
e
Δδ (H ) / ppm
/ ppm
/ ppm
ppm
The precursor amino acids and solvents were purchased from Reanal
Laborvegyszer Kft., while the other reagents from VWR International
(
(
(
R)-1
R)-4
R)-5
overlapping resonances
overlapping resonances
overlapping resonances
overlapping resonances
0.046
0.059
0.077
0.059
0.004
0.006
0.008
0.008
0.021
0.018
0.037
0.021
Kft. FTIR spectra were obtained on a Bruker Alpha FTIR equipped with
diamond ATR. For known compounds 1H NMR, 13C NMR and 19F NMR
(
2R,3S)-6
spectra were recorded on Bruker Avance 250 spectrometer using 5
mm inverse 1H/ C/ P/ F probehead at room temperature. New
compounds were characterized by a 600 MHz Varian DDR NMR
spectrometer equipped with a 5 mm inverse‐detection gradient
13
31 19
(IDPFG) probehead. Standard pulse sequences and processing
routines available in VnmrJ 3.2C/Chempack 5.1 were used.
1H(400 MHz) and 19F(376 MHz) spectra for enantiomeric excess
determination were recorded on a Varian Mercury Plus spectrometer.
Chemical shifts (δ) are given in ppm units relative to the internal
1
standards: TMS (δ = 0.00 ppm for H). For obtaining high resolution
mass spectrometric data an Orbitrap
Q Exactive Focus Mass
Spectrometer equipped with electrospray ionization (Thermo Fischer
Scientific, Waltham, MA, USA) was used. Melting points were
determined by Boetius-micro melting point apparatus, are
uncorrected. Optical rotations were determined on a Carl Zeiss
Polamat A polarimeter with a 1 dm cell at 25 °C.
Experimental
Figure 6. Selected 1H NMR resonances of (RS)-prasugrel (bottom)
General Procedure for the Synthesis of Methyl α-Hydroxycarboxylates
and the same resonances after the addition of equimolar amounts of
The solution of α-amino acid (0.15 mol) in 1 M H
cooled to 0 °C, and the solution of NaNO (62.1 g, 0.9 mol) in water
150 ml) was added dropwise, in a rate, that the temperature
2
SO
4
(300 ml) was
various α-(nonafluoro-tert-butoxy)carboxylic acids in C
6
6
D .
2
(
Conclusions
remained under 5 °C. The solution was stirred at 0 °C for 1 h, then at
rt overnight. NaCl (33.0 g) was added to the mixture, and it was
extracted with diethyl ether (4×90 ml). The combined organic phases
In conclusion, α-(nonafluoro-tert-butoxy)carboxylic acids (1, 4, 5, 6)
exhibit chiral recognition ability toward racemic amines in apolar
solvents. The enantiomers of the racemic analyte and the chiral
solvating agents formed diastereomeric complexes, which were
were dried over Na
pressure. The remaining oil was dissolved in methanol (110 ml), then
cooled to 0 °C, and SOCl (11.61 ml, 0.15 mol) was added dropwise.
2
SO
4
, and the solvent was removed under reduced
distinguishable by NMR spectroscopy. In CDCl
difference of methine hydrogens allow the discrimination of the
enantiomers present, while in C the methyl hydrogens are also
3
the chemical shift
2
The reaction mixture was stirred at rt overnight then the solvent was
removed under reduced pressure and the crude product was purified
by vacuum distillation.
6
D
6
distinguishable. The measured Δδ values were 0.004-0.086 ppm,
which are comparable to those of known chiral NMR shift reagents.
Methyl (S)-(+)-2-hydroxypropanoate (S)-7
L-alanine (15.00 g, 0.17 mol) was reacted according general
procedure to give 7.08 g (43 %) colorless liquid.
5
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
B.p. 144 °C. (Lit. b.p. 144-145 °C [30]) [α]546 = +11.6 (c = 2, methanol).
25
The solution of methyl α-hydroxycarboxylate (40 mmol),
triphenylphosphine (15.7 g, 60 mmol) and nonafluoro-tert-butanol
(15.1 g, 64 mmol) in diethyl ether (100 ml) was cooled to 0 °C, and
the solution of DIAD (12.1 g, 60 mmol) in diethyl ether (50 ml) was
added dropwise. The reaction mixture was stirred at rt for 12 h. Then
the solvent was removed under reduced pressure and the remaining
oil was steam-distilled. The organic phase of the distillate was
1H NMR (250 MHz, CDCl
) δ 4.29 (q, J(H,H) = 6.9 Hz, 1H, CH), 4.18 (br
3
3
3
s, 1H, OH), 3.77 (s, 1H, COOCH
13C
NMR (63 MHz, CDCl
3.0 (COOCH ), 20.6 (CH
N ; calc. 104.0473).
3
), 1.41 (d, J(H,H) = 6.9 Hz, 3H, CH
3
CH).
3
)
δ
176.5 (COOCH
3
), 67.2 (CH),
+
5
3
3
CH). HR-MS: 104.0474 ([M+NH
4
] ,
+
C
4
H
12
O
3
Methyl (S)-(+)-2-hydroxy-3-methylbutanoate (S)-8
separated and dried over Na
vacuum distillation.
2
SO . The crude product was purified by
4
L-valine (15.23 g, 0.13 mol) was reacted according general procedure
to give 7.05 g (41 %) colorless liquid.
B.p. 58-59 °C / 15 Torr. (Lit. b.p. 61-62 °C / 15 Torr. [25]) [α]54625
=
Methyl (R)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-
yl)oxy)propanoate (R)-11
1
+
18.0 (c = 2, methanol). H NMR (250 MHz, CDCl
3
) δ 4.12 (br s, 1H,
), 2.05 (dtd,
CH),
CH), 0.84 (d, J(H,H) = 6.9 Hz, 3H,
) δ 175.7 (COOCH ), 75.5 (CH),
CH), 19.1 (CH CH), 16.4 (CH CH).
+; calc. 132.0787).
3
OH), 4.04 (d, J(H,H) = 3.6 Hz, 1H, CH), 3.77 (s, 1H, COOCH
3
(S)-7 (3.30 g, 32 mmol) was reacted according general procedure to
give 6.60 g (59 %) colorless liquid.
3J(H,H) = 13.8 Hz, 3J(H,H) = 6.9Hz, 3J(H,H) = 3.6 Hz, 1H, (CH
3 2
)
3
3
25
1
0
.99 (d, J(H,H) = 6.9 Hz, 3H, CH
3
B.p. 131-135 °C. [α]546 = +32.9 (c = 2, methanol). H NMR (250 MHz,
CDCl ),
) δ 4.71 (q, 3J(H,H) = 6.6 Hz, 1H, CH), 3.78 (s, 3H, COOCH
1.54 (d, 3J(H,H) = 6.8 Hz, 3H, CH CH). 13C NMR (63 MHz, CDCl
δ 171.0 (COO), 120.5 (q, 1J(C,F)
292 Hz, CF ), 74.4 (CH),
CH
3
CH). 13C NMR (63 MHz, CDCl
3
3
3
3
52.8 (COOCH
3
), 32.4 ((CH
3
)
2
3
3
3
3
)
+
HR-MS: 132.0787 ([M+H] , C
6
H O
13 3
=
3
52.4 (COOCH
3
), 19.7 (CH
3
CH). 19F NMR (235 MHz, CDCl
) δ -70.68
3
+
F
+; calc. 322.0251).
Methyl (S)-(+)-2-hydroxy-4-methylpentanoate (S)-9
(CF
3
). HR-MS: 322.0249 ([M+H] , C
8
H
8
O
3
9
L-leucine (15.00 g, 0.11 mol) was reacted according general
procedure to give 7.63 g (46 %) colorless liquid.
Methyl (R)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-
yl)oxy)-3-methylbutanoate (R)-12
B.p. 76-77 °C / 15 Torr. (Lit. b.p. 69-70 °C / 8 Torr. [25]) [α]546 = +10.6
25
1
3
(
c = 2, methanol). H NMR (250 MHz, CDCl
3
) δ 4.19 (t, J(H,H) = 6.6 Hz,
(S)-8 (7.05 g, 53 mmol) was reacted according general procedure to
give 6.60 g (37 %) colorless liquid.
1
H, CHOH), 3.75 (s, 3H, COOCH
3
), 3.01 (br s, 1H, OH), 1.83 (m, 1H
),
), 0.91 (d, 3J(H,H) = 2.4 Hz, 3H,
) δ 176.7 (COO), 69.4 (CHOH),
), 1.53 (dd, 3J(H,H) = 6.8 Hz, 3J(H,H) = 6.8 Hz, 2H, CH
B.p. 153-157 °C. [α]546 = +16.7 (c = 2, methanol). H NMR (250 MHz,
25
1
CH(CH
3
)
2
2
3
) δ 4.41 (d, 3J(H,H) = 5.3 Hz, 1H, CHCOOCH
), 3.77 (s, 3H,
0
.93 (d, J(H,H) = 2.2 Hz, 3H, CHCH
3
CDCl
COOCH
6H, CH
). 13C NMR (63 MHz, CDCl
= 292.4 Hz, CF
3
3
). 13C NMR (63 MHz, CDCl
) 1.65-1.78 (m, 3H, CH2CH(CH
)
2
), 0.94 (d, J(H,H) = 6.2 Hz,
3
CHCH
3
3
3
3
1
5
2.8 (COOCH
1.9 (CHCH
3
), 43.8 (CH
2
), 24.7 (CH(CH
3
)
2
), 23.6 (CHCH
3
),
3
3
) δ 170.5 (COOCH
3
), 120.5 (q, J(C,F)
+
+; calc. 146.0943).
2
2
3
). HR-MS: 146.0942 ([M+H] , C
7
H
15
O
3
3
) 82.5 (CHCOOH), 77.2 (q, J(C,F) = 120.5 Hz, C(CF
3
)
3
),
) δ -
+; calc. 350.0564).
3
3.1 (CH(CH
3
)
2
), 18.0 (CH
3
), 17.8 (CH
3
). 19F NMR (235 MHz, CDCl
3
+
7
0.80 (CF ). HR-MS: 350.0559 ([M+H] , C10
3
H
12
O
F
3 9
Methyl (2S,3S)-(+)-2-hydroxy-3-methylpentanoate (2S,3S)-10
L-isoleucine (15.00 g, 0.11 mol) was reacted according general
procedure to give 10.27 g (61 %) colorless liquid.
Methyl (R)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-
yl)oxy)-4-methylpentanoate (R)-13
B.p. 75-77 °C / 15 Torr. [α]546 = +5.3 (c = 2, methanol). 1H NMR
2
5
(R)-9 (7.00 g, 48 mmol) was reacted according general procedure to
give 13.125 g (75 %) colorless liquid.
3
(250 MHz, CDCl
3
) δ 4.08 (d, J(H,H) = 3.8 Hz, 1H, CH(OH)), 3.77 (s, 3H,
25
1
COOCH
3
), 2.71 (s, 1H, OH), 1.88 – 1.71 (m, 1H, CH, CHCH
3
), 1.43 – 1.13
B.p. 170-172 °C. [α]546 = +18.1 (c = 2, methanol). H NMR (250 MHz,
3
3
) δ 4.65 (t, 3J(H,H) = 6.2 Hz, 1H, CHCOOCH
), 3.76 (s, 3H,
(m, 2H, CH
2
), 0.96 (d, J(H,H) = 6.9 Hz, 3H, CHCH
3
), 0.88 (t, J(H,H) =
CDCl
3
3
3
7
7
1
.4 Hz, 3H, CH
5.2 (CHOH), 52.6 (COOCH
2.1 (CH
2
CH
3
). 13C NMR (63 MHz, CDCl
3
) δ 175.8 (COOCH
), 24.1 (CH ), 15.7 (CH
+; calc. 146.0943).
3
),
),
COOCH
3
) 2.24-2.08 (m, 1H, CH(CH
3
)
2
), 1.01 (d, J(H,H) = 7.0 Hz, 3H,
3
). 13C NMR (63 MHz, CDCl
)
3
), 39.5 (CHCH
3
2
3
CH
3
), 0.98 (d, J(H,H) = 6.9 Hz, 3H, CH
3
3
+
1
3
). HR-MS: 146.0941 ([M+H] , C
7
H O
15 3
δ 169.6 (COOCH
2J(C,F) = 293 Hz, C(CF
19F NMR (235 MHz, CDCl
+; calc. 364.0721).
3
), 120.5 (q, J(C,F) = 292 Hz), 82.5 (CHCOOH), 75.4 (q,
3
)
3
), 33.1 (CH(CH
3
)
2
), 18.0 (CH
3
), 17.8 (CH ).
3
General procedure for the synthesis of methyl α-(nonafluoro-tert-
3
) δ -70.54 (CF
3
). HR-MS: 364.0718 ([M+H]+,
butoxy)carboxylates
C
11
H
14
O
3 9
F
6
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
25
M.p. 57-60 °C. [α]546 = +21.6 (c = 2, methanol). IR (ATR): 424, 536,
634, 660, 727, 893, 966, 987, 1008, 1023, 1144, 1184, 1247, 1374,
1
1
434, 1650, 1720, 2977. H NMR (600 MHz, CDCl
3
) δ 9.20 (br s, 1H,
3
3
Methyl
(2R,3S)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)-
COOH), 4.46 (d, J(H,H) = 4.9 Hz, 1H, CHCOOH), 2.22 (td, J(H,H) =
3
3
propan-2-yl)oxy)-3-methylpentanoate (2R,3S)-14
13.7 Hz, J(H,H) = 6.9 Hz, 1H, CH(CH
3
)
2
), 1.05 (d, J(H,H) = 7.1 Hz, 3H,
3
(
2S,3S)-10 (9.05 g, 65 mmol) was reacted according general
CH
3
), 1.04 (d, J(H,H) = 6.9 Hz, 3H, CH
3
). 13C NMR (125 MHz, CDCl
293 Hz, CF ), 84.1 (CH),
). 19F NMR (235 MHz, CDCl
3
)
procedure to give 14.846 g (63 %) colorless liquid.
δ 176.8 (COOH), 122.7 (q, 1J(C,F)
34.3 (CH(CH ), 20.2 (CH ), 20.0 (CH
70.45 (CF ). HR-MS: 336.0418 ([M+H] , C
=
3
25
1
B.p. 168-171 °C. [α]546 = +17.6 (c = 2, methanol). H NMR (250 MHz,
3
)
2
3
3
3
) δ -
) δ 4.38 (d, 3J(H,H) = 4.5 Hz, 1H, CHOC(CF
)
3
), 3.64 (s, 3H,
+
F
3 9
+; calc. 336.0408).
CDCl
COOCH
3
3
3
9
8
H O
3
3
), 1.76, 1.46, 1.03 (m, 3H, CH
2
CH), 0.88 (d, J(H,H) = 7.0 Hz, 3H,
CH
3
CH), 0.81 (d, 3J(H,H) = 7.4 Hz, 3H, CH
3
CH). 13C NMR (63 MHz,
), 81.7 C(CF ),
CH), 24.9 (CH ),
). 19F NMR (235 MHz, CDCl
) δ -70.6(CF ).
+; calc. 364.0721).
(R)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yl)oxy)-
4-methylpentanoic acid (R)-5
1
CDCl
3
) δ 169.7 (COOCH
5.2 (CHOC(CF
4.5 (CH ), 11.9 (CH
3
), 120.5 (q, J(C,F) = 294 Hz, CF
3
3 3
)
7
1
3
)
3
), 52.3 (COOCH
3
), 39.8 (CH
2
2
(R)-13 (9.00 g, 25 mmol) was reacted according general procedure.
The acidified mixture was an emulsion, which was extracted with
diethyl ether (3 × 30 ml), and the combined organic phases were
3
3
3
3
+
HR-MS: 364.0715 ([M+H] , C11
H
14
O
F
3 9
dried over Na
2
SO
4
.
The solvent was removed under reduced
General procedure for the synthesis of α-(nonafluoro-tert-
butoxy)carboxylic acids
pressure, to give 4.87 g (56 %) colorless liquid.
2
5
[α]546 = +13.0 (c = 2, methanol). IR (ATR): 539, 637, 728, 969, 1014,
1154, 1248, 1371, 1471, 1735, 2966. 1H NMR (600 MHz, CDCl
δ 9.54 (br s, 1H, COOH), 4.54 (d, 3J(H,H) = 4.1 Hz, 1H, CHCOOH),
To the solution of α-(nonafluoro-tert-butoxy)carboxylate (30 mmol)
3
)
in CH
2
Cl 1.2 M NaOH in MeOH (75 ml) was added, and the mixture
2
was stirred at rt for 2 h. Then the solvent was removed under
reduced pressure, and the remaining material was dissolved in water
1.94(m, 1H, CHCH
Hz, 3H,CHCH
MHz, CDCl
(CH), 34.3 (CH(CH
CDCl ) δ -70.55 (CF
350.0564).
2
), 1.63 and 1.21 (m, 2H, CH
2
), 1.03 (d, 3J(H,H) = 7.1
CH
). 13C NMR (125
), 84.1
3
3
), 0.94 (t, J(H,H) = 7.4 Hz, 3H, CH
2
3
(35 ml). The solution was cooled to 0 °C and acidified with cc HCl to
3
) δ 176.8 (COOH), 122.7 (q, 1J(C,F) = 293 Hz, CF
3
pH=2. After standing for an additional 1 h crystals were filtered,
washed with water and dried. The crude product was recrystallized
from hexane.
3
)
2
), 20.2 (CH
3
), 20.0 (CH
3
). 19F NMR (235 MHz,
+
F
+; calc.
3
3
). HR-MS: 350.0573 ([M+H] , C10
H O
10 3
9
(
R)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-
yl)oxy)propanoic acid (R)-1
R)-11 (6.08 g, 19 mmol) was reacted according general procedure to
give 4.57 g (79 %) white solid material.
(2R,3S)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-
yl)oxy)-3-methylpentanoic acid (2R,3S)-6
(
(2R,3S)-14 (12.00 g, 33 mmol) was reacted according general
procedure to give 8.18 g (71 %) white solid material.
2
5
25
M.p. 52-54 °C. [α]546 = +33.6 (c = 2, methanol). IR(ATR): 487, 539,
M.p. 52-55 °C. [α]546 = +8.3 (c = 2, methanol). IR (ATR): 430, 480,
633, 696, 724, 769, 838, 885, 967, 989, 1030, 1091, 1122, 1156, 1227,
512, 538, 672, 727, 903, 966, 1004, 1106, 1139, 1177, 1248, 1372,
249, 1357, 1423, 1457, 1733, 2955. 1H NMR (600 MHz, CDCl
)
1434, 1641, 1717, 2974. H NMR (600 MHz, CDCl
1
) δ 8.45 (br s, 1H,
1
3
3
3
3
3
δ 10.22 (br s, 1H, COOH), 4.75 (q, J(H,H) = 6.8 Hz, 1H, CH), 1.61 (d,
COOH), 4.62 (t, J(H,H) = 6.0 Hz, 1H, CHCOOH), 1.74 (dd, J(H,H) = 5.6
3J(H,H) = 6.8 Hz, 3H, CH
22.7 (q, 1J(C,F) = 293 Hz, CF
235 MHz, CDCl ) δ -70.9 (CF ). HR-MS: 308.0105 ([M+H] , C
). 13C NMR (125 MHz, CDCl
) δ 178.4 (COOH),
). 19F NMR
Hz, J(H,H) = 2.6 Hz, 2H, CH
3
), 1.68-1,77 (m, 1H, CH(CH ) ), 0.90 (d,
3
3
2
3
2
1
3
), 76.0 (CH), 22.0 (CH
3
3J(H,H) = 2.3 Hz, 3H, CH
13C NMR (125 MHz, CDCl
CF
), 79.0 (q, 2J(C,F) = 293 Hz, C(CF
2.9 (CH(CH ), 21.5 (CH ), 21.4 (CH
70.73 (CF ). HR-MS: 350.0574 ([M+H] , C10
3
), 0.89 (d, 3J(H,H) = 2.3 Hz, 3H, CH
).
3
+
+;
1
(
3
3
H
7 5
O
3
F
9
3
) δ 174.3 (COOH), 119.1 (q, J(C,F) = 293 Hz,
calc. 308.0095).
3
3
)
3
), 75.2 (CHCOOH), 41.5 (CH
). 19F NMR (235 MHz, CDCl
) δ -
+; calc. 350.0564).
2
),
2
3
)
2
3
3
3
+
(R)-(+)-2-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yl)oxy)-
3
H
10
O
F
3 9
3-methylbutanoic acid (R)-4
(R)-12 (5.95 g, 17 mmol) was reacted according general procedure to
NMR experiments
give 4.12 g (72 %) white solid material.
NMR sample solutions were made as follows: 0.027 mmol carboxylic
acid was dissolved in 600 μl of deuterated solvent. After H and 19F
1
7
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
NMR measurements, 0.027 mmol amine were added, and the NMR
spectra were recorded. All measurements were carried out at 298 K.
[9]
J. M. Seco, E. Quiñoá, R. Riguera, ‘Assignment of the Absolute
Configuration of Polyfunctional Compounds by NMR Using Chiral
Derivatizing Agents’, Chem. Rev. 2012, 112, 4603–4641.
10] H. Dodziuk, A. Ejchart, O. Lukin, M. O. Vysotsky, ‘ H and 13C NMR and
Molecular Dynamics Study of Chiral Recognition of Champhor
Enantiomers by α-Cyclodextrin’, J. Org. Chem. 1999, 64, 1503-1507.
1
[
Supplementary Material
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/MS-number.
[11] M. Bernabé, J. Jiménez-Barbero, M. Martin-Lomas, S. Penadés, C. Vicent,
Chiral recognition of 1-O-allyl- and 1-O-benzyl-D- and -L-myo-inositol
‘
by cyclomalto-hexaose and -heptaose (α- and β-cyclodextrin)’,
Carbohydr. Res. 1990, 208, 255-259.
Acknowledgements
[
[
12] Z. Aturki, C. Desiderio, L. Mannina, S. Fanali, ‘Chiral separations by
capillary zone electrophoresis with the use of cyanoethylated-β-
cyclodextrin as chiral selector’, J. Chromatogr. A 1998, 817, 91-104.
The authors thank the National Research, Development and
Innovation Office (K115764) for supporting this work. This work was
partially supported by the János Bolyai Research Scholarship of the
Hungarian Academy of Sciences and by the Bolyai+ New National
Excellence Program (grant number: ÚNKP-19-4-SE-53) of the Ministry
of Human Capacities (Sz.B.).
2
13] Y. Nakatsuji, Y. Nakahara, A. Muramatsu, T. Kida, M. Akashi, ‘Novel C -
Symmetric Chiral 18-Crown-6 Derivatives with Two Aromatic Sidearms
as Chiral NMR Discriminating Agents’, Tetrahedron Lett. 2005, 46, 4331-
4335.
[
14] D. J. Chadwick, I. A. Cliffe, I. O. Sutherland, R. F. Newton, ‘The Formation
of Complexes between Aza Derivatives of Crown Ethers and Primary
Alkylammonium Salts. Part 7. Chiral Derivatives of Aza Crown Ethers’, J.
Chem. Soc. Perkin Trans. 1 1984, 1707-1717.
Author Contribution Statement
A.N. and T.Cs. designed experiments and made preliminary syntheses.
A.N. and D.Sz. improved them to the laboratory scale and wrote the
manuscript. A.N., Sz.B. and Zs.G. made the one- and two-dimensional
NMR measurements along with chiral discrimination experiments,
Zs.G. recorded the IR and MS spectra, resp. J.R. surveyed fluorous
literature. All authors commented on manuscript.
[
[
[
15] Thomas J. Wenzel James D. Wilcox, ‘Chiral Reagents for the
Determination of Enantiomeric Excess and Absolute Configuration Using
NMR Spectroscopy’, Chirality 2003, 15, 256-270.
16] C. A. R. Baxter, H. C. Richards, ‘Substituted 1,2,3,4-Tetrahydroquinolines.
Measurement of Optical Purity by Nuclear Magnetic Resonance
Spectroscopy’, Tetrahedron Lett. 1972, 3357-3358.
17] H. Navrátilová, ‘Use of S‐Mosher Acid as a Chiral Solvating Agent for
Enantiomeric Analysis of Some trans ‐ 4‐ (4‐ Fluorophenyl) ‐ 3‐
substituted‐1‐methylpiperidines by Means of NMR Spectroscopy’,
Chirality 2002, 731-735.
References
[
1]
L. He, T. E. Beesley, T. E. ‘Applications of Enantiomeric
GasChromatography: A Review’, J. Liquid Chromatography and Related
Technologies 2005, 28, 1075–1114.
[
[
18] K. Y.Y. Lao, D. J. Hodgson, B. Dawson, P. H. Buist, ‘A Micromethod for the
Stereochemical Analysis of Fatty Acid Desaturase-Mediated
Sulfoxidation Reactions’, Bioorg. Med. Chem. Lett. 2005, 15, 2799-2802.
19] W. H. Pirkle, S. D. Beare, ‘Optically Active Solvents in Nuclear Magnetic
Resonance Spectroscopy. IX. Direct Determinations of Optical Purities
and Correlations of Absolute Configurations of α-Amino Acids’, J. Am.
Chem. Soc. 1969, 91, 5150-5155.
[
2]
E. Badaloni, W. Cabri, A. Ciogli, R. Deias, F. Gasparrini, F. Giorgi, A.
Vigevani, C. Villani, ‘Combination of HPLC ‘Inverted Chirality Columns
Approach” and MS/MS Detection for Extreme Enantiomeric Excess
Determination Even in Absence of Reference Samples. Application to
Camptothecin Derivatives’ Anal. Chem. 2007, 79, 6013–6019.
[
3]
I. Ilisz, R. Berkecz, A. Antal, ‘Application of chiral derivatizing agents in
the high-performance liquid chromatographic separation of amino acid
enantiomers: A review’, J. Pharm. Biomed. Anal. 2008, 47, 1–15.
L. G. Blomberg, H. Wan, ‘Determination of enantiomeric excess by
capillary electrophoresis’, Electrophoresis 2000, 21, 1940–1952.
D. Leung, S. O. Kang, E. V. Anslyn, ‘Rapid determination of enantiomeric
excess: a focus on optical approaches’ Chem. Soc. Rev. 2012, 41, 448–
[
20] M. de Moragas, E. Cervelló, A. Port, C. Jaime, A. Virgili, B. Ancian,
‘Behavior of the 9-Anthryl-tert-butylcarbinol as a Chiral Solvating Agent.
Study of Diastereochemical Association by Intermolecular NOE and
Molecular Dynamics Calculations’, J. Org. Chem. 1998, 63, 8689-8695.
21] T. Csóka, A. Nemes, D. Szabó, ‘Synthesis of optically active α-
[
[
4]
5]
[
[
[
(
nonafluoro-tert-butoxy)carboxylic acids’, Tetrahedron Lett. 2013, 54,
730–1733.
1
479.
22] A. Nemes, T. Csóka, Sz. Béni, V. Farkas, J. Rábai, D. Szabó, ‘Chiral
Recognition Studies of α-(Nonafluoro-tert-butoxy)carboxylic Acids by
NMR Spectroscopy’, J. Org. Chem. 2015, 80, 6267–6274.
[
[
[
6]
7]
8]
T. J. Wenzel, ‘Discrimination of Chiral Compounds Using NMR
Spectroscopy’, John Wiley & Sons, Inc. Hoboken, New Jersey, 2007.
D. Parker, ‘NMR determination of enantiomeric purity’, Chem. Rev. 1991,
23] M. del Pilar García-Santos, E. Calle, J. Casado, ‘Amino Acid Nitrosation
Products as Alkylating Agents’, J. Am. Chem. Soc. 2001, 123, 7506-7510.
91, 1441–1457.
J. M. Seco, E. Quiñoá, R. Riguera, ‘The Assignment of Absolute
Configuration by NMR’, Chem. Rev. 2004, 104, 17–117.
8
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
[
24] F. Degerbeck, B. Fransson, L. Grehn, U. Ragnarsson, ‘Synthesis of 15N-
Labelled Chiral Boc-Amino Acids from Triflates: Enantiomers of Leucine
and Phenylalanin’, J. Chem. Soc. Perkin Trans. 1 1993, 11-14.
[
[
25] M. Poterała, J. Plenkiewicz, ‘Synthesis of new chiral ionic liquids from α-
hydroxycarboxylic acids’, Tetrahedron Asy. 2011, 22, 294-299.
26] O. Mitsunobu, M. Yamada, ‘Preparation of Esters of Carboxylic an
Phosphoric Acids via Quaternary Phosphonium Salts’, Bull. Chem. Soc.
Jpn. 1967, 40, 2380-2382.
[
[
[
27] K. C. Kumara Swamy, N. N. Bhuvan Kumar, E. Balaraman, K. V. P. Pavan
Kumar, ‘Mitsunobu and Related Reactions: Advances and Applications’,
Chem. Rev. 2009, 109, 2551-2651.
28] A.-M. Bálint, A. Bodor, Á. Gömöry, K. Vékey, D. Szabó, J. Rábai,
‘Mitsunobu synthesis of symmetrical alkyl and polyfluoroalkyl secondary
amines’, J. Fluorine Chem. 2005, 126, 1524-1530.
29] D. Szabó, A.-M. Bonto, I. Kövesdi, Á. Gömöry, J. Rábai, ‘Synthesis of novel
lipophilic and/or fluorophilic ethers of perfluoro-tert-butyl alcohol,
perfluoropinacol and hexafluoroacetone hydrate via
reaction: Typical cases of ideal product separation’, J. Fluorine Chem.,
005, 126, 641–652.
a Mitsunobu
2
[
30] M.J. O'Neil, ‘The Merck Index - An Encyclopedia of Chemicals, Drugs,
and Biologicals. Cambridge’, UK: Royal Society of Chemistry, 2013., p.
1130.
9
This article is protected by copyright. All rights reserved.

Helvetica Chimica Acta
10.1002/hlca.202000081
HELVETICA
Entry for the Table of Contents
Twitter
Four chiral α-(nonafluoro-tert-butoxy)carboxylic acids were synthesized from natural α-amino acids (alanine, valine, leucine and
isoleucine), and tested in NMR experiments as shift reagents.
10
This article is protected by copyright. All rights reserved.